Cancer researchers have found that certain types of dental X-rays significantly increased the incidence of the most common type of brain tumor in the United States: meningioma.

The study authors offer the caveat that their subjects likely received X-rays as kids that delivered a much higher dose of radiation than those used today. But the numbers are still alarming: they found that people diagnosed with meningiomas were more than twice as likely to have undergone so-called bitewing X-rays—the kind that tend to hurt like a mother when the dentist makes you bite down on a hard, pointy piece of cardboard with your molars.

Exposure to ionizing radiation via atomic bombs and radiation therapy has been previously linked to meningiomas, but Dr. Elizabeth Claus, the study's lead author and a professor at the Yale School of Medicine in New Haven, Connecticut, says dental X-rays are the most common source of exposure. Her results are similar to other, smaller dental X-ray studies.

Claus and her team looked at 1,433 people in Connecticut and the San Francisco Bay Area who had been diagnosed with intracranial meningioma between May 2006 and April 2011 when they were between 20 and 79. They compared them with 1,350 tumor-free individuals similar in age, sex and geographic location.

Those with tumors were twice as likely to report having received bitewing X-rays. Also, yearly or more frequent bitewings were associated with a 40 to 90 percent higher risk of having a brain tumor.

Another type of X-ray called "panoramic" images taken before kids were 10 increased the risk by up to five times.

Confusingly, the researchers found no association between full-mouth X-rays—which include multiple bitewings—and the tumors. Dr. Alan Lurie, president of the American Academy of Oral and Maxillofacial Radiology told Reuters:

They found a small risk (from) a pair of bitewings, but not a full mouth series, which is multiple bitewings. That inconsistency is impossible to understand to me.